1. Field of the Invention
This invention relates generally to electromagnetic machines and, more specifically, to a variable-speed, variable-frequency synchronous machine and system that can operate as a motor or generator/alternator, wherein the primary input and output parameters, such as rotational speed, frequency, and voltage, are fully decoupled from one another.
2. Description of the Related Art
As illustrated in FIG. 2a, many conventional electronically-controlled, variable-speed, alternating current (AC) motor drive systems include a conventional electromechanical machine, typically an induction motor 10. Such systems are taught by Knox, U.S. Pat. No. 4,491,778, and many others. On a first order basis, motor speed is proportional to its power input frequency. Motor 10 is made to run at a commanded speed, different from what is allowed by the prime power source, by interposing a variable-voltage, variable-frequency (VVVF) controller/power processor 12 between it and its power source 14.
Developments in power electronic components, embedded computers, and control understanding have enabled controller improvements, and allowed for many different control algorithms for specific applications. Such systems are not universally applied because of losses in the controller and the unavailability of controllers for large systems, since these controllers must process all of the power required by the system load. To improve efficiency, several system types have been proposed for part-time variable speed operation and part-time full-speed operation, with switching methods to support them.
Systems, such as that illustrated in FIG. 2b, have been proposed that obviate the need for a series-connected, full-power controller. U.S. Pat. No. 4,039,909, issued to Baker, discloses a variable-speed, variable-frequency, AC machine that does not require a full-power controller interposed between the machine and its power source. The Baker patent teaches a three-phase AC electromechanical machine that functions as a variable-speed, fixed-frequency (VSFF) motor 5; wherein the stator and rotor may be independently excited. Independent, three-phase rotor excitation produces a rotating magnetic field vector, which is independent of the rotor structure. Adjusting the angular velocity of the rotor magnetic field vector, by adjusting the frequency of its applied excitation power, may be used to control the machine's speed, through its interaction with the fixed-speed stator magnetic field vector. The rotor field is controlled by a three-phase VVVF exciter circuit 16, which is not in the source/load power path of the machine, i.e., is not coupled between motor 18 and its power source 20. The exciter is connected to the three-phase set of rotor windings by slip rings and brushes. Proper closed loop control through the exciter can duplicate any of the above VVVF control algorithms. The Baker patent further teaches that this basic operational method can also be applied to generators.
The Baker patent also proposes operation with DC power by interposing chopper circuitry so that the machine continues to operate on AC while used in DC systems. There is no direct operation as a motor or alternator with DC inputs or outputs.
U.S. Pat. No. 4,994,684, issued to Lauw, et al., discloses a specialized, three-phase exciter to be used with a "Baker" type machine. It controls the output of a dynamic rotor field alternator (with slip rings and brushes) to provide fixed-frequency power to a terrestrial power distribution grid, independent of the speed of the turbine driving the AC generator/alternator. The exciter is patented on the basis that it is a unique design, because it uses specialized control algorithms, particularly appropriate to the utility power grid application.
It is also known in the art that machine rotor windings need not be directly connected through slip rings and brushes, but rather can be coupled through a separate, secondary set of stator and rotor windings, which function as a rotary transformer. U.S. Pat. No. 5,028,804 also issued to Lauw teaches that a secondary set of conventional, three-phase rotor and stator windings can couple AC excitation power to the rotor of an alternator, to replace Baker's slip rings and brushes. The rotor power is connected to the primary three-phase rotor windings on a per-phase basis, to provide the controlled rotor excitation for output power frequency control, using the dynamic rotor field principle. A third Lauw Patent, U.S. No. 5,239,251 discloses that this basic brushless, rotating transformer approach can be used in a wound rotor motor which has Baker type dynamic rotor field control of the output speed.
While it eliminates the need for the in-line controller that processes 100% of the power used by most of today's variable-frequency, variable-speed, AC motor and generator systems, the implementation of the dynamic rotor field principle utilized in the Baker and Lauw patents suffers from a number of disadvantages. In the configurations disclosed in the Baker patents, both the motors and generators include wound, three-phase rotors energized through slip rings and brushes. This type of electromechanical machine is significantly more complex and expensive than an equivalent induction machine. The improvements to such machines suggested in the Lauw patents eliminate the slip rings and brushes, but at the cost of still more stator and rotor complexity. Therefore, neither configuration has found wide commercial acceptance.
U.S. Pat. No. 5,430,362, issued to Carr et al., discloses the use of an electromagnetic generator in its "reverse" direction as a motor to start an engine-driven power source (specifically, an aircraft auxiliary power unit (APU)) to eliminate the need for a separate starting motor. This machine is a compound synchronous machine having a permanent magnet armature portion and a separately-excited main armature. Brushless excitation is provided through a rotating, single-phase to three-phase transformer. The coupled AC power is rectified to DC to provide a constant magnetic field, with its vector fixed with respect to the rotor structure. When this machine is used as a variable-speed motor to bring the prime mover up to speed for starting, the rotor is energized with a fixed field through a brushless, rotating transformer-rectifier path. The three-phase stator windings that normally provide the power output are energized by a separate, full-power, three-phase, VVVF controller (called the "APU Start Converter"). The control algorithms for the VVVF controller are specifically designed to be compatible with unique machine switching, acceleration, and speed requirements when it is operated in the starting motor mode.
Although the above-described Baker and Lauw machines have radially-wound, AC rotors, solenoid-wound rotors which use DC excitation are also known. Many conventional automotive alternators have pole pieces which enclose a single, DC, solenoid-wound rotor coil. The pole pieces are magnetized from each end of the coil. They carry the magnetic field that the winding induces and are interleaved with opposite-end/opposite-polarity poles, such that north and south poles are alternated around the circumference of the rotor. The coil rotates with the rotor, and rotor electrical connections are made with slip rings and brushes.
The present invention overcomes the above-described problems and deficiencies in the art in the manner described below.